Myrtille O.J.Y. Hunault*1

Myrtille O.J.Y. Hunault
Graduated from both the ESPCI ParisTech and the University Pierre and Marie-Curie in Paris (2011). She did her PhD (2011-2014) research at the IMPMC in the University Pierre and Marie Curie under the supervision of Prof G. Calas and Dr. M. Hérold, on the colouring role Co2+ in medieval stained glass windows. After a 1-year postdoctoral fellowship to study the colours of the 15th century rose of the Sainte-Chapelle of Paris and a second postdoctoral fellowship in the group of Prof. F. de Groot (Utrecht, the Netherlands) on the fundamentals of x-ray spectroscopy of oxides, she is since 2017 scientist on the MARS beamline of the SOLEIL synchrotron (France). Her research focuses on the speciation and colouring role of transition metal and actinides ions in glasses and crystals, using optical and synchrotron based x-ray spectroscopies and theoretical calculations.  

Myrtille O.J.Y. Hunault*1, Gérald Lelong2, Laurence Galoisy2, Laurent Cormier2, Georges Calas2
1SOLEIL Synchrotron, l’Orme des merisiers Saint-Aubin, Gif-Sur-Yvette, France
2Sorbonne Université, Institut de Minéralogie de Physique des Matériaux et Cosmochimie, Paris France

Before it became the fuel of nuclear power plants, uranium has been largely used as an efficient colouring agent in glasses and glazes. Today, the reactivity of uranium is still intensively studied for chemical and environmental purposes. It is also used as a surrogate of transuranic elements in nuclear waste storage matrices. In glasses and ceramics, uranium is mainly present in the forms of U(VI) and U(IV), with minor amounts of U(V). Silicate glasses show uranyl sites UO22+ with an original geometry, based on shorter distances between U and equatorial oxygens than in the most uranyl complexes found in crystals and aqueous solutions. This intermediate speciation of U(VI) in glasses is also illustrated by their color, which may vary from green to brown, depending on glass composition. Uranium speciation has been investigated since years in complex technological borosilicate glasses using XANES, EXAFS and neutron/x-ray diffraction data. However, since the preliminary work of A. Paul,[1] there have been no investigation of the simple alkali borate glasses, despite they show a large diversity of cation sites as a function of the presence of various structural superunits.We present the X-ray absorption study of the speciation of uranium(VI) in low (10%) and high (30%) lithium borate glasses based on a crystal chemistry approach.[2] Comparison of uranyl bond distances obtained by EXAFS with distances found in borate crystals reveals that in the low lithium borate glass uranyl is present as hexagonal bipyramids with six equatorial oxygen ligands. This local environment was never observed in any other oxide glass. We show that the increase of the lithium content induces the decrease of the equatorial coordination number. The associated uranyl bond elongation suggests the influence of the alkali cations in relation with drastic changes in the structure of the borate network. Comparison with the cases of transition metal ions will the discussed.   [1] Paul, A. Acid-Base Concepts in Relation to the Structure of Borate and Silicate Glasses. Trans. Indian Ceram. Soc.1969, 28 (3), 63–81. [2] Hunault, M. O. J. Y.; Lelong, G.; Cormier, L.; Galoisy, L.; Solari, P.-L.; Calas, G. Speciation Change of Uranyl in Lithium Borate Glasses. Inorg. Chem.2019, 58 (10), 6858–6865.